A sensor of this kind is already known from European Patent Application EP-0 334 441.
This sensor comprises a sensor body formed by a housing and a conductive front surface, with an intermediate piece for insulating the conductive surface from the housing. This sensor also comprises a coaxial cable for connecting the conductive surface to a device for measuring the capacitance.
The conductive surface is to be aligned with the internal surface of the casing of a turbo-engine in the field of aeronautics. The measurement of the capacitance occurring between the conductive surface of the sensor and the end of the rotating blades of the engine reveals the clearance existing, in the dynamic state, between these extremeties and the casing.
This sensor also enables measurement of the temperature at the point of measurement of the capacitance. To this end, the core of the coaxial cable consists of the two wires of a thermocouple, the welding point of which is in electrical and thermal contact with the conductive front surface of the sensor body. The wires of the thermocouple thus transport the electric measurement signal for the temperature as well as the electric measurement signal for the capacitance.
The complete device is suitable to withstand the very high temperatures prevailing within the turbojet. The coaxial cable is of the type shielded by a mineral insulator. The sheath of the coaxial cable is formed by a metallic cylinder having a very small diameter, i.e. in the order of from 1 to 2 min. The core is insulated from the sheath by a compacted mineral insulator. The sheath is cut at one end of the cable so as to expose the welding point and the wires of the thermocouple over a small length.
The welding point of the thermocouple is sealed to the conductive surface reinforced by a metallic electrode. The intermediate insulation piece is sealed on the one side to the housing and to the other side to the electrode. Moreover, the housing is sealed to the metallic sheath of the coaxial cable for tightness of the sensor assembly. Moreover, all parts are made of a refractory material.
The described device nevertheless has drawbacks. First of all, its manufacture necessitates the use of a large number of different elements, which is expensive. The pieces must be very exactly adjusted, which is also expensive.
Secondly, the number of seals is large, and the seals are susceptible to vibrations. However, depending on the type of engine in which the sensor is used, the vibrations may be very different and very destructive in given cases.